clean model when plugin was removed

Author: thurka

visualvm/graalvm/src/org/graalvm/visualvm/graalvm/libgraal/MemoryModel.java

@@ -177,6 +177,7 @@ synchronized void cleanup() {
         if (jvm != null && monitoredDataListener != null) {
             jvm.removeMonitoredDataListener(monitoredDataListener);
         }
+        connection = null;
     }
 
     void addChangeListener(ChangeListener listener) {

visualvm/graalvm/src/org/graalvm/visualvm/graalvm/libgraal/MemoryViewPlugin.java

@@ -52,6 +52,11 @@ protected void willBeAdded() {
         model.initialize();
     }
 
+    @Override
+    protected void removed() {
+        model.cleanup();
+    }
+
     MemoryModel getModel() {
         return model;
     }